The present invention is directed to a spent pulping liquor recovery process in which the effective capacity of the spent pulping liquor recovery furnace is significantly increased by adding to an unoxidized strong spent pulping liquor stream, prior or subsequent to concentration thereof, a predetermined amount of partially oxidized, evaporated spent pulping liquor having a substantially reduced heating value. The partially-oxidized, concentrated spent pulping liquor formed thereby is capable of being combusted in the furnace without the addition of auxiliary heating fuel.
In conventional pulping of lignocellulose employing a chemical pulping liquor, as schematically depicted in FIG. 1, weak spent pulping liquor denoted "WSL" (total solids of about 15-20% for alkaline weak pulping liquor), containing various by-product materials, is formed during the pulping operations. These by-product materials include inorganic material such as pulping chemicals, and organic materials such as lignocellulosic derivative compounds produced during alkaline pulping. The weak spent pulping liquor stream is evaporated to produce a strong spent pulping liquor designated "SSL" (45-50% by weight total solids), and is subsequently concentrated to a high total solids level of about 60-70% by weight. The concentrated spent pulpihg liquor product (CPSL) is then fired in a conventional recovery furnace so that the organic material is combusted, and the inorganic material and the heat of combustion are largely recovered.
Many commercial pulping facilities are limited in their pulp output because they are operating at the maximum capacity of their recovery furnace to combust spent pulping liquor. However, if this maximum capacity is exceeded, an elevated temperature profile will exist throughout the recovery furnace which will fuse entrained inorganic material present in the flue gas and cause fireside plugging of the recovery furnace convective sections. Therefore, if this total combustive heat release per unit of pulp production in recovery limited furnaces were significantly reduced, the pulp production rate could be increased.
If the spent pulping liquor introduced into the furnace can be modified so that the total heat released therein is lowered, firing of additional spent pulping liquor can result. This reduction in total heat release can be accomplished by lowering the heating value of the spent pulping liquor introduced into the furnace. The heating value is defined as energy evolved during combustion. In one approach, organic materials in the respective weak or strong spent pulping liquor streams are oxidized using air and/or oxygen to decrease the heating value thereof.
Various treatment systems have been employed by the prior art in an attempt to oxidize spent liquor to various extents. In U.S. Pat. No. 3,714,911, the entire weak spent liquor stream is subjected to wet air oxidation wherein two pounds of water per pound of air is evaporated prior to combustion in a recovery furnace. Although this is said to eliminate the need for multi-effect evaporation of the partially-oxidized spent pulping liquor, it actually produces a material having a lower heating value than the minimum value required for supporting combustion in a recovery furnace without the addition of auxiliary fuel.
Other attempts to oxidize spent pulping liquor include mild wet air oxidation, and oxidation with molecular oxygen, of sodium sulfide in the spent pulping liquor to sodium thiosulfate for odor control. See U.S. Pat. Nos. 3,709,975; 3,873,414; 4,073,727; 3,549,314; and 3,567,400.
In a process designed to eliminate the need for a recovery furnace, substantially all of the heating value of the spent pulping liquor is removed by complete flameless oxidation of all of the inorganic and organic materials present therein. See
U.S. Pat. Nos. 2,824,058 and 2,903,425.
When an entire weak or strong spent liquor stream is oxidized so that its heating value is substantially reduced, and thus oxidized liquor is ultimately concentrated to the requisite level for introducing same into the recovery furnace, the liquor viscosity thereof will be increased to such an extent that it will not flow, and in some cases will actually be solidified. The above oxidation of weak black liquor has the further drawback of fouling the multiple-effect evaporators and causing excess foaming therein during the formation of strong black liquor.